The present invention relates to ignition systems for internal combustion engines and more particularly to a control system for controlling the duration time of ignition sparks.
During low speed and low load operation of a spark-ignition engine, the ignition of the air-fuel mixture is difficult due to a large quantity of the residual burned gases. Moreover, the recent trend toward using exhaust emission controls and lean air-fuel mixtures has made it increasingly important to improve the ignition performance during the periods of low speed and low load operation.
It has been discovered that if the duration time of sparks is increased to about 3 msec, for example, as a means for overcoming the foregoing difficulty, the ignition capacity can be increased greatly under such operating conditions.
However, this increased duration time of sparks is disadvantageous in that not only the duration time is wasted during the periods of high speed or high load operation where the ignition performance is good, but also the rate of wear of the spark electrodes is increased considerably. Thus, apparently it has been impossible to accomplish both the increased ignition capacity and the avoidance of the wear of the plug electrodes at a time.
However, the present inventors have discovered that the wear of the electrodes of spark plugs is mostly attributable to the discharge after the piston has passed the top dead center on the compression stroke and that the discharge occuring before the top dead center on the compression stroke practically contributes nothing to the wear of the electrodes.
This is considered to be due to the fact that the occurrence of discharge in the presence of a high pressure, high temperature and flame promotes the rate of wear of the spark plug electrodes.
Also, the studies made by the inventors have shown that the discharge occurring after the top dead center on the compression stroke makes practically no contribution to the ignition. As a result, there has existed a need for an ignition system so designed that the spark is not allowed to continue beyond the top dead center on the compression stroke and that the spark is maintained as far as possible for the periods of low speed and low load operation.
To meet these requirements, as disclosed in the specification of U.S. Pat. No. 3,896,776, a system has been proposed in which when the top dead center (TDC) position is reached, the primary winding is energized to cut off the secondary discharge. In this system, a first voltage pulse train and a second voltage pulse train are produced so that in response to the positive-going transition of the first voltage pulse train, the second voltage pulse train including a much greater number of pulses than the first voltage pulse train is computed to determine the starting point of ignition and in response to the negative-going transition of the first voltage pulse train the primary winding of the ignition coil is energized to cut off the secondary discharge. Thus, the primary winding is held "OFF" during the time interval between the production of a spark at the ignition starting point and the termination of the spark at the TDC and the primary winding is held "ON" throughout the other periods. In accordance with this characteristic diagram the calculation of the ON and OFF periods of the primary winding shows that the corresponding ON and OFF periods for every 1/2 engine revolution are respectively 4.3 ms and 0.7 ms with the OFF/ON ratio of 16% at the engine speed of 6000 rpm, 8.9 ms and 1.1 ms with the ratio of 12% at 3000 rpm and 48.3 ms and 1.7 ms with the ratio of 3.5% at 600 rpm. As a result, there is a disadvantage that if the ignition control is performed according to this control system throughout the speeds of an engine, the ignition coil generates heat and eventually it is burned out. In order to avoid the disadvantage, it is considered to deenergize the primary winding at the expiration of a predetermined time after the primary winding has been energized to cut off the secondary discharge. But it causes another disadvantage of producing a harmful high voltage in the secondary winding upon the deenergization of the primary winding. Relating to an ignition system for solving such problems, a U.S. Pat. copending application Ser. No. 264,693 has been filed on May 18, 1981 and assigned to the assignee of the present application.